RMs and mixtures of RMs can be used to make optical films, like compensation, retardation or polarisation films, e.g. for use as components of optical or electrooptical devices like LC displays, through the process of in-situ polymerisation. The optical properties of the films can be controlled by various factors, such as mixture formulation or substrate properties.
The films are usually prepared by coating a solution of an RM or of an RM mixture on a substrate, removing the solvent, aligning the RMs into the desired orientation, and polymerising the coated and aligned RMs in situ by exposure to heat or actinic radiation, for example to UV radiation, and optionally removing the polymerised film from the substrate.
However, coating of a substrate with a solution containing RMs can lead to a build up of static charge if the solution is not conducting. This can lead to an electrostatic discharge by arcing, and, if the solvent is flammable, result in a fire or explosion. This hazard can be reduced by engineering solutions such as the use of tinsel and electrostatic neutralization bars. However, the rapid pumping of a non-conductive flammable fluid to a coating head can also lead to electrostatic discharge.
For example, a typical manufacturing method for such polymerised RM film products is roll to roll processing of a plastic substrate, in which coating of a reactive mesogen solution is an integral step. During the production steps there is the potential for build-up of electric charge, which must be discharged to prevent the danger of uncontrolled discharge, or the potential for defects or damage of the final product.
Moreover, the build up of static charge can lead to unwanted effects in polymerised optical films such as uneven coating, which can be visually observed and is often referred to as “mura”.
It is generally known that such electrostatic charge build up can be mitigated if conducting solvents are used (see e.g. Matthew R. Giles in Organic Process Research & Development 2003, 7, 1048-1050), however, this greatly reduces the choice of solvents available to the formulator. This is especially disadvantageous because RMs are not commonly soluble in polar solvents. Besides, it is also possible that such solvents are not compatible with the substrates onto which the solution should be coated.
This invention has the aim of providing improved RM formulations and methods which enable the preparation of polymer films with reduced, or even without, build up of static charge, in order to avoid drawbacks and risks like electrostatic discharge by arcing, uneven coatings and optical defects like mura, while retaining the advantageous RM and film properties such as good coatability, good alignment and high durability. Other aims of the invention are immediately evident to the expert from the following description.
The inventors of the present invention found that these aims can be achieved by adding certain conductive additives to the RM formulation. Thereby the static build up when coating the mixture on a substrate is reduced, whilst other properties such as good coatability, alignment and durability are retained.
WO 2003/083523 discloses that polar additives can be added to liquid crystal (LC) mixtures, however their role is to induce electro-hydrodynamic instability in a bistable LC mode. The additives are therefore not polymerisable. WO 2008/110342 also describes the use of highly polar additives to enhance the properties of an RM formulation, but in this case, the additives are magnetic or paramagnetic particles. Also the formulations described in WO 2008/110342 are designed for inkjet printing rather than solution coating. WO 2008/110316 discloses the use of similar polar pigments in RM formulations that can be processed to prepare polarisation selective scattering devices for security devices. WO 2008/110317 describes the preparation of inkjettable RM formulations, but wherein the polar additives are pigmented or polymer additives. US 2011/0240927 describes a formulation that contains a direactive RM and a polar additive, and can be processed to make a transparent conducting layer as an alternative to the ITO electrodes that are commonly used as electrodes for LC display (LCD) manufacture. However, in this case, the amount of RM is limited to 20% and the formulation does not give a film that acts as a birefringent retardation film. In this case, the additives are PEDOT/PSS (poly-3,4-ethylenedioxy-thiophene/polystyrene sulphonate), which are not soluble in the solvents commonly used for RM formulations. However, the above-mentioned documents do neither disclose nor suggest the RM formulations, methods and uses as claimed in the present invention.